1. Field of the Invention
The present invention relates to a rotary magnetic head device used for a video tape recorder, which will be referred to as “VTR” in this specification, hereinafter. More particularly, the present invention relates to a rotary magnetic head device in which a rotary drum having a magnetic head is supported by a support shaft, which is a stationary shaft, via a bearing.
2. Description of the Related Art
According to Japanese Unexamined Patent Publication No. 58-180828 (Prior Art Example 1), there is a description about a fluid bearing cylinder device having a rotary side unit and stationary side unit. This Prior Art Example 1 describes the fluid bearing cylinder device as follows. A rotary transformer is attached to the stationary side unit, and a permanent magnet is attached to the rotary side unit. When this permanent magnet is opposed to the rotary transformer, a predetermined attraction force is generated in the direction of thrust. A thrust fluid bearing, which generates a bearing force in the direction opposite to the attraction force, is arranged being put on a forward end face of a stationary shaft having grooves. When the attraction force of the permanent magnet and the weight of the rotary side unit are balanced with the bearing force of the fluid bearing, a direction of thrust of the rotary side unit is subjected to positional regulation. Further, the following descriptions are made. The rotary side unit is supported by a stationary shaft via a radial direction fluid bearing which bears a radial load, and the radial direction fluid bearing is formed by spiral grooves made on an outer circumferential face of the stationary shaft. Furthermore the following descriptions are made. The thrust direction fluid bearing and radial direction fluid bearing are formed when lubricant is held on the sliding faces of those bearings.
On the other hand, according to Japanese Unexamined Patent Publication Nos. 4-95206 (Prior Art Example 2) and 4-26911 (Prior Art Example 3), there is a description of a rotary head device having a rotary drum pivotally supported by a support shaft. According to Japanese Examined Patent Publication No. 3-30009 (Prior Art Example 4), there is a description of a dynamic fluid bearing used as a thrust bearing by which abrasion of the thrust member can be reduced as small as possible. Other than Prior Art Example 1 described above, Japanese Unexamined Patent Publication Nos. 8-235545 (Prior Art Example 5) and 1-149213 (Prior Art Example 6) and also according to Japanese Examined Patent Publication No. 6-82453 (Prior Art Example 7) make a description of suppressing the occurrence of abrasion of a thrust bearing.
In Prior Art Examples 1 to 7 described above, Prior Art Example 1 discloses a structure in which the thrust bearing is fixed to the rotary side unit with screws. Prior Art Example 2 discloses the same structure. On the other hand, according to Prior Art Examples 5 to 7, there are no specific descriptions of the means for fixing the thrust bearing to the rotary drum side. According to the structure described in Prior Art Example 4, the thrust bearing is attached to the rotary sleeve by a holding ring composed of a spring sheet. According to the structure described in Prior Art Example 3, the thrust bearing composed of a spherical member is held inside the sleeve member by adjustment screws.
However, in Prior Art Example 1, a force (pressing force) to be balanced with the support force of a fluid bearing is obtained by the attraction force of a permanent magnet. Therefore, the following problems may be encountered.
(1) According to the characteristic of a permanent magnet, the permanent magnet attracts a magnet body. Therefore, the permanent magnet attracts fine magnetic powder mixed in dust. Accordingly, there is a possibility that the initial attraction characteristic of the permanent magnet is impaired. In order to prevent the occurrence of this problem, it is necessary to take specific countermeasure against the problem.
(2) An attraction force of the permanent magnet changes being in inverse proportion to the square of a distance from the permanent magnet to an object to be attracted. Therefore, in the case of Prior Art Example 1 described before, it is necessary to highly accurately adjust a distance from the permanent magnet to the rotary transformer, which is troublesome taking time and labor.
(3) A rotary transformer is attached to the rotary side or stationary side unit, and the permanent magnet is attached to the rotary side unit. Therefore, it is indispensable to incorporate the permanent magnet inside the fluid bearing cylinder. Accordingly, the efficiency of assembling work is impaired and the productivity is deteriorated.
(4) Since the thrust direction fluid bearing and radial direction fluid bearing are formed by feeding lubricant into grooves, it is necessary to conduct maintenance on those fluid bearings so as to maintain the initial performance of them. Further, since the permanent magnet is incorporated inside the fluid bearing cylinder as described above, the maintenance work takes much time and labor.
As described in Prior Art Examples 1 to 7, when the thrust bearing is attached by fasteners such as attaching screws, holding rings or adjusting screws, it is necessary to provide parts to be used as the fasteners. Further, it is necessary to provide a process in which the thrust bearing is attached to the rotary drum and sleeve member, which raises the entire production cost.
Although a sliding face of the thrust bearing is lubricated at all times, since the thrust bearing is usually made of metal, they always demands improvements in the abrasion resistance property. For the above reasons, in order to improve the abrasion resistance property of the sliding face, it is preferable that the thrust bearing is made of synthetic resin, which is called engineering plastic, the sliding property and abrasion resistance property of which are high. However, when the above structure is adopted, since it is common that synthetic resin is not electrically conductive, it is difficult to connect the rotary drum with ground by using the thrust bearing as one of the electrical passage composing members.